Many electrical protection devices employ various means to obtain trip curves that define the point at which current to the protected device will be interrupted. Some electrical protective devices use an I2*t trip curve, which is preferred over an I*t trip curve because it provides a better model of the protected circuit or device. Thermal overload relays employ a resistive heating element to model electrical heating in a protected device. This resistive heating inherently results in overload trip curves with I2*t characteristics. The I2*t characteristic is especially desirable in motor protective devices, such as solid state overload relays (SSOLRs), because it can closely model I2*R motor winding heating. Typically SSOLRs model motor heating in either analog or digital electronics, by performing some sort of signal processing to convert signals representing line currents into input for a thermal model. Electronic thermal modeling introduces an obstacle to achieving I2*t overload curve characteristics by requiring a squaring operation during the processing of these signals. There are several electronic solutions that achieve an I2*t trip curve characteristic. However, each solution has its own inherent advantages and disadvantages and presently all require a significant power budget that limits their application to only those overload protection devices that can provide sufficient power for the solution to work.
In self powered overload protection devices, a low power budget presents a very large obstacle for achieving the desired trip curves, such as one having the I2*t trip curve characteristics. This obstacle is significant enough that self powered overload protection devices with the desirable I2*t overload trip curve are relatively unknown. The less desirable I*t trip curve is employed in all but the most expensive self powered overload protection devices. Therefore, a method for composing desirable trip curves, such as the I2*t trip curve, using very little power and in a less expensive manner would be desirable.